Mode filter for an electromagnetic waveguide

ABSTRACT

A substantially rigid mode filter of circular cross-section for use with electromagnetic waveguides includes an inner tubular layer of insulating material and, extending over the outer surface of the insulating layer throughout its length, a plurality of mutually spaced, circumferentially continuous rings or at least one helix with closely spaced turns of an electrically conductive coating. Preferably the mode filter is rendered substantially rigid by a reinforcing body of electrically insulating material, e.g., resin bonded glass fibre, which surrounds the ring or helix. Each end of the mode filter is preferably shaped to form a socket in which an end of a waveguide will fit.

United States Patent 1 Kidner et al. Nov. 6, 1973 MODE FILTER FOR AN 3,573,681 4 1971 Miller 333 95 R ELECTROMAGNETIC.WAVEGUIDE FOREIGN PATENTS OR APPLICATIONS [75] Inventors: Henry Gerald Kidner, Chislehurst; 820,455 11/1951 Germany 333/98 R David Alexander Taylor, Kenton, 866,960 2/1953 Germany 333/95 M both of England OTHER PUBLICATIONS Assigneei f l Insulated Callelldefs Cables Rose, C. F. P. Research Models of Helix Waveguides,

Limited, Lonon, England Bell System Tech. Jr. 5-1958, pp. 679-687 [22] Filed: Feb. 1, 1972 Barlow, H. E. M., A Method of Changing the Dominant Mode in a l-lollow Metal Waveguide & lts Appli- PP A cation to Bends, IEE v01. 10613, Supp. 13, 1959 pp.

[30] Foreign Application Priority Data P E R d l h v R r rzmary xammeru o p o Inec Feb. 2, 1971 Great BI'ItaIn 3,706/71 Assistant Examiner wm. H. Punter 52 0.5. CI 333/95 R, 333/98 M V [51] Int. Cl. ..l-I0.1p 1/16, H01p 3/12 [58] Field of Search 333/98 M, 98 R, 95 R [571 ABSTRACT A substantially rigid mode filter of circular cross- [56] R f Cit d section for use with electromagnetic waveguides in- UNITED cludesan inner tubular layer 0f insulating material and, extending over the outer surface of the insulating layer 3,492,607 1/1970 Effemey 333/95 R throughout Its length, a plurahty of mutually spaced,

circumferentially continuous rings or at least one helix 3,149,295 9/1964 Grebe n 333/98 R with closely spaced turns of an electrically conductive 2 44 023 5 953 Bemct 33 93 R coating. Preferably the mode filter is rendered substan- 2,564,007 8/1951 Hochgraf 333/98 R tially rigid by a reinforcing body of electrically insulat- 2,695,255 11/1954 1 Avery 333/95 R ing material, e.g., resin bonded glass fibre, which surfarbowiakg rounds the ring or helix. Each end of the mode filter is awson i preferably shaped to form a socket In which an end of a 311101001 11/1963 Unger 333 95 R 5 Claims, 6 Drawing Figures MODE FILTER FOR AN ELECTROMAGNETIC WAVEGUIDE This invention relates to mode filters forelectromagnetic waveguides of circular cross-section for use in effecting communication over a long distance by conveying microwave signals along the waveguides arranged continuously along an appropriate transmission path.

in the propagation of the lowest circular electric mode (TE using electromagnetic waveguides of circular cross-section it is the general practice to incorpo rate in the transmission path at spaced locations along its length mode filters to suppress the propagation of unwanted modes introduced byirregularities in construction or curvature of the waveguide or waveguides. In transmission paths built up of electromagnetic wave guides of circular cross-section each comprising a continuous circumferential electrically conductive layer a common form of mode filter used for this purpose is a helix waveguide comprising an inner layer of closely wound turns of insulated copper wire covered with a lossy sheath.

It is an object of the present invention to provide, for use with electromagnetic waveguides of circular crosssection, a novel mode filter which is cheap to manufacture but is of high dimensional accuracy.

According to the invention a substantially rigid mode filter of circular cross-section includes an inner tubular layer of insulating material and,,extending over the outer surface of the insulating layer throughout its whole length, a plurality of mutally spaced, circumferentially continuous rings of an electrically conductive coating or at least one helix with closely spaced turns of an electrically conductive coating.

The required rigidity of the mode filter preferably is provided by a reinforcing body of electrically insulating material which surrounds the electrically conductive rings or helix and which may comprise a plurality of layers of resin bonded glass fibre or other suitable reinforcing material. At least one layer of lossy material, for instance glass fibre tape containing graphite or other electrically conducting material, is preferably provided between the electrically conductive rings or helix and the reinforcing body. The mode filter may have an outer conductive screen, for instance a thin steel jacket or a braided wire sleeve.

Mode filters in accordance with the invention may be used in combination with a plurality of substantially rigid electromagnetic waveguides of circular crossa section each having a continuous circumferential electrically conductive layer extending throughout its length to build up a transmission path which has mode filters distributed at intervals along its length and which, by virtue of the substantially rigid construction of the waveguides and mode filters, is of high dimensional accuracy.

The invention therefore includes a transmission path built up of a plurality of substantially rigid electromagnetic waveguides of circular cross-section each having a continuous circumferential electrically conductive layer extending throughout its length, at least one pair and preferably each pair of adjacent waveguides being connected end to end by a mode filter in accordance with the invention.

Adjacent waveguides and mode filters are preferably connected by spigot and socket connections. Such a spigot and socket connection may be effected by a separate unit making spigot and socket connections with a waveguide and mode filter but preferably the mode filter may itself constitute the connection unit for mak ing a spigot and socket connection between adjacent waveguides and for this purpose each end of the mode filter may be shaped to form a socket in which an end of a waveguide fits. Alternatively the ends of each waveguide may be shaped to form sockets into each of which an end of a mode filter constituting a connector unit fits.

In accordance with a further aspect of the invention we provide a method of forming a substantially rigid mode filter in accordance with the invention which-includes the steps of applying a continuous circumferential layer of electrically conducting metal to a mandrel of circular cross-section and, before or after removing the mandrel from the electrically conductive layer so formed, removing parts or a part of the layer to form a plurality of mutually spaced, circumferentially continuous rings or atleast one helix with closely spaced turns of electrically conducting metal.

The dimensions of the continuous rings or of the helix or helices so formed will be such that the mode filter will suppress to at least a substantial extent the propagation of unwanted modes in a transmission path of which the mode filter fon'ns a part.

The inner tubular layer of insulating material may be preformed or may be applied as a coating to the internal surface of the mode filter and in both cases preferably it is inserted into or applied to the mode filter after the mandrel is removed but it may be initially applied over the mandrel and may itself constitute the outer part of the mandrel on which the electrically conductive layer is formed-In both cases a thin layer of material that will facilitate easy removal of the mandrel, for instance a coating of graphite, is preferably applied to the mandrel before forming the conductive layer.

The continuous circumferential electrically conductive layer may be formed by electro plating, spraying or vacuum deposition of a metal, e.g., copper, or by any other suitable method. The mutually spaced rings or helix of electrically conducting metal may be formed by initially applying to the mandrel a plurality of mutually spaced circumferentially continuous rings or at least one helix with closely spaced turns of a resist from which the electrically conductive metal can be easily removed, e.e., by etching, or, iii-the case where the inner tubular layer of insulating material is not provided within the mode filter until the mandrel is removed, the continuous electrically conductive layer may be formed and, after the mandrel has been removed, appropriate parts or an appropriate part of the conductive layer removed by machining to form the mutually spaced rings or helix electrically conducting metal.

A layer of electrically insulating material, for example resin bonded glass fibre, will normally be applied over the rings or helix of electrically conducting metal, and a layer or layers of lossy material, for instance carbon impregnated resin bonded glass fibre, applied over the insulating layer. The radial thickness of the lossy material will be such that in conjunction with the continuous rings or helix or helices the required impedance for the suppression of unwanted modes of propagation is provided. Layers of electrically insulating material, for example resin bonded glass fibre, to provide therequired rigidity, will normally also be applied over the lossy material, such layers being applied, and when necessary cured, before the mandrel is removed. An outer screen is preferably applied overall.

The mandrel, which is preferably of metal, may be of cylindrical cross-section throughout its length but, where each end of the mode filter to be formed is to constitute a socket, the mandrel may be divided transversely into two parts each of appropriate increased cross-section at one end.

The invention will be further illustrated by a description, by way of example, of two forms of mode filter in accordance with the invention and of two transmission paths in which mode filters in accordance with the invention are connected, with reference to the accompanying drawings in which:

FIG. 1 is a sectional view of one form of mode filter with the inner tubular insulating layer omitted;

FIG. 2 is a fragmental diagrammatic sectional side view of a wall of the mode filter shown in FIG. 1, drawn on an enlarged scale;

FIG. 3 is a sectional side view of a second form of mode filter with the inner tubular insulating layer omitted;

FIG. 4 is a side view of a mandrel for forming the mode filter shown in FIG. 3, shown partly in section and partly in elevation;

FIG. 5 is a fragmental diagrammatic representation of one form of transmission path incorporating mode filters as shown in FIG. 3, and

FIG. 6 is a fragmental diagrammatic representation of a second form of transmission path incorporating mode filters as shown in FIG. 1.

The mode filter 1 shown in FIGS. 1 and 2 is of circular cross section and has an overall length of 0.33 m (1.08 ft), an internal diameter of 5 cms (1.97 in) and an overall wall thickness of 5.2 mm (0.20 in). It comprises an inner layer 2 of polyethylene having a radial thickness of 0.2 mm (0.0079 in); 2,750 rings 3 of copper coating each having a radial thickness of 5 [.L m (0.00020 in) and an axial length of 0.1 mm (0.004 5): the spacing between adjacent rings being 0.02 mm (0.00079 in); a layer 5 of resin bonded glass fibre having a radial thickness of 0.1 mm (0.004 in); a lossy layer 6 of glass fibre tape rendered semi-conductive by the incorporation of graphite, the lossy layer having a radial thickness of 0.9 mm (0.036 in); a body 7 built up of layers of resin bonded glass fibre to a radial thickness of approximately 4.0 mm (0.16 in), the body rendering the mode filter substantially rigid, and, overlying the reinforcing body 7, an outer screen comprising a thin steel jacket (not shown).

The mode filter 11 shown in FIG. 3 includes 2,750 rings 13 of copper coating and is of similar construction to the mode filter 1 shown in FIGS. 1 and 2 except that, at each of its ends, the reinforcing layer extends beyond the end of the rings 13 and its bore is enlarged to form a socket 18 having an internal diameter such that a substantially rigid waveguide of circular-cross-section having an overall diameter equal to that of the mode filter will make a tight spigot and socket joint in an end of the mode filter.

The mode filter 11 is formed on a mandrel 22 of circular cross-section which is shown in FIG. 4 and which is divided transversely into two separable parts 23, 24. The part 23 has an externally screwthreaded spigot 25 which screws into a tapped hole 26 in the complementary end of the part 24. At the ends of the parts 23, 24

remote from the spigot 25 and tapped hole 26 the overall diameter of the mandrel is increased to form end portions 27, 28 on which the sockets 18 of the mode filter are formed. When the mode filter 11 has been assembled on the mandrel 22 in the manner hereinbefore described the mandrel is readily removed by unscrewing the two parts 23, 24 and withdrawing them from the ends of the mode filter.

In using mode filters 11 as shown in FIG. 3 to make the transmission path shown in FIG. 5 each adjacent pair of a plurality of substantially rigid electromagnetic waveguides 10 of circular cross-section, each having a continuous circumferential electrically conductive layer extending throughout its length, is connected end to end by a mode filter 11, the neighbouring ends of adjacent waveguides each making a spigot and socket joint with the sockets 18 of the interposed mode filter. The transmission path so formed is substantially rigid throughout its length.

In the second form of transmission path shown in FIG. 6 there is interposed between each adjacent pair of a plurality of substantially rigid electromagnetic waveguides 10 of circular cross-section a mode filter 1 of the form shown in FIG. 1, the overall diameter of each mode filter being substantially equal to that of the waveguides. Neighbouring ends of a waveguide 10 and mode filter l fit into and make spigot and socket joints with opposite ends of a tubular connector unit 20 of insulating material. As in the transmission path shown in FIG. 5 the transmission path shown in FIG. 6 is substantially rigid throughout its length.

Mode filters in accordance with the invention have the important advantage of high dimensional accuracy because the dimensions of the bore of the mode filter are determined by the dimensions of the mandrel which can be accurately ground to the desired size.

What we claim as our invention is:

1. For use with electro-magnetic waveguides of circular cross-section, a substantially rigid mode filter of circular cross-section comprising an inner tubular layer of insulating material; a plurality of mutually spaced, circumferentially continuous rings of an electrically conductive coating or at least one helix with closely spaced turns of an electrically conductive coating, extending over the outer surface of the insulating layer throughout its whole length; and a reinforcing body of electrically insulating material which surrounds, and extends beyond each end of the electrically conductive rings or helix and renders the mode filter substantially rigid, the projecting ends of the reinforcing body each being shaped to form a socket with a substantially smooth internal surface in which an end of a waveguide can make a spigot and socket joint.

2. A mode filter as claimed in claim 1, wherein the reinforcing body comprises a plurality of layers of resin bonded glass fibre.

3. A mode filter as claimed in claim 1, wherein at least one layer of lossy material is provided between the electrically conductive rings or helix and the reinforcing body.

4. A transmission path built up of a plurality of substantially rigid electro-magnetic waveguides of circular cross-section each having a continuous circumferential electrically conductive layer extending throughout its length, at least one pair of adjacent waveguides being connected end to end by a substantially rigid mode filter of circular cross-section including an inner tubular ing body each being shaped to form a socket which has a substantially smooth internal surface, the neighboring ends of said pair of adjacent waveguides making spigot and socket joints in the ends of the mode filter.

5. A transmission path as claimed in Claim 4, wherein each pair of adjacent waveguides is connected end to end by a substantially rigid mode filter as claimed in claim 1. 

1. For use with electro-magnetic waveguides of circular crosssection, a substantially rigid mode filter of circular crosssection comprising an inner tubular layer of insulating material; a plurality of mutually spaced, circumferentially continuous rings of an electrically conductive coating or at least one helix with closely spaced turns of an electrically conductive coating, extending over the outer surface of the insulating layer throughout its whole length; and a reinforcing body of electrically insulating material which surrounds, and extends beyond each end of the electrically conductive rings or helix and renders the mode filter substantially rigid, the projecting ends of the reinforcing body each being shaped to form a socket with a substantially smooth internal surface in which an end of a waveguide can make a spigot and socket joint.
 2. A mode filter as claimed in claim 1, wherein the reinforcing body comprises a plurality of layers of resin bonded glass fibre.
 3. A mode filter as claimed in claim 1, wherein at least one layer of lossy material is provided between the electrically conductive rings or helix and the reinforcing body.
 4. A transmission path built up of a plurality of substantially rigid electro-magnetic waveguides of circular cross-section each having a continuous circumferential electrically conductive layer extending throughout its length, at least one pair of adjacent waveguides being connected end to end by a substantially rigid mode filter of circular cross-section including an inner tubular layer of insulating material, a plurality of mutually spaced, circumferentially continuous rings of an electrically conductive coating, or at leasT one helix with closely spaced turns of an electrically conductive coating extending over the outer surface of the insulating layer throughout its whole length, and a reinforcing body of electrically insulating material which surrounds, and extends beyond each end of the electrically conductive rings or helix and renders the mode filter substantially rigid, the projecting ends of the reinforcing body each being shaped to form a socket which has a substantially smooth internal surface, the neighboring ends of said pair of adjacent waveguides making spigot and socket joints in the ends of the mode filter.
 5. A transmission path as claimed in Claim 4, wherein each pair of adjacent waveguides is connected end to end by a substantially rigid mode filter as claimed in claim
 1. 